Propane fractionation of lubricating oil stocks



W. N. AXE ET AL Filed April 4. 1949 PROPANE FRACTIONATION OF LUBRICATING OIL STOCKS Dec. 23, 1952 INVENTORS W. N. AXE

. DE VAULT A T TOR/V5 KS Patented Dec. 23, 1952 UNITED STATES PATENT OFFICE PROPANE FRACTIONATION F LUBRICATING OIL STOCKS Application April 4, 1949, Serial No. 85,416

Claims.

This invention relates to propane fractionating of lubricating oil stocks. In one of its more specific aspects it relates to a method for the treatment of low asphalt-containing lubricating oil stocks for the separation and recovery of the lubricating oil fractions. In a still more specific aspect it relates to a process for the separation and recovery of lubricating oil fractions from low asphalt-content crude lubricating oil stocks by fractionation with liquid propane wherein asphaltic constituents are not precipitated and caked in the fractionation apparatus.

The asphalt content of a crude oil may conveniently be defined as the per cent of residue having a maximum ASTM D243-36 penetration of 100 at 77 F. from a vacuum distillation of the crude oil. For Oklahoma City crude oils this asphaltic residue constitutes about 2 per cent by weight of the crude oil.

By the term low asphalt content-crude oil we mean crude oils containing a maximum of about 5 per cent asphalt by weight as determined by the above mentioned ASTM method. Such a low asphalt-containing crude oil may be distinguished from conventional asphaltic crude oils such as California crudes which contain from 12 to 65 per cent asphalt, and from Wyoming and Arkansas crude oils which contain about 30 per cent asphalt.

An object of our invention is to devise a process in which a low asphalt-containing reduced crude lubricating oil stock is fractionated for the separation and recovery of lubricating oil constituents. Another object of our invention is to devise a method for the separation and recovery of lubricating oil fractions from such low asphaltcontent crude lubricating oil stocks by fractionation with liquid propane without the formation of hard asphaltic deposits in the oil fractionation apparatus. Still other objects and advantages of the process of our invention will be obvious to those skilled in the art upon reading the following disclosure, which, taken with the attached drawing, forms a part of this specification.

The separation of liquid asphaltic materials from petroleum oils and more particularly from reduced crude oils by means of propane is old in the art. Such operations are usually carried out at moderate temperatures in the range of about 70 F. to 140 F. in the case of Mid-Continent residua to yield asphaltic oils to the extent of about 2 to per cent depending on conditions of temperature and propane to oil ratio. In addition to this conventional type of deasphalting operation, it has-been recognized for many years L'II that a fractionating effect can be'realizedin-the case of propane-oil mixtures subjected to temperatures in the region of F. up to the critical temperature of propane or higher. This fractionation effect is a direct result of the unique properties of propane-oil systems wherein high molecular Weight components and the more aromatic and naphthenic components of the oil are separated first'from solution with increasing temperature. Thus, in countercurrent towers using a stripping section and a temperature differential in the rectifying section, a combination of the effects of low-temperature distillation andsolvent extraction may be achieved. However, 'due to difficulties imposed in operating in close proximity to the critical temperature of propane, commercial exploitation of thisprinciple in the production of the-relatively light SAE 40 to 70 grades of oil was unknown until comparatively recent times.

Inasmuch .as the fractionation of residual stocks into close-cut'fractions in the SAE 30 to 50 gradeshas certain obvious technical and com- .mercial advantages, extensive investigation of this type of separation has disclosed problems not hitherto reported in the prior art. The most troublesome problem is that presented by the separation of a third asphaltic phase in the countercurrent tower which renders the process inoperative in :the :case of many representative Mid-Continent residual stocks. Thus in the propane fractionation of threewellknown and representative reducedprudesirom widely separated Oklahoma fields, two were found not to be continuouslysamenable to this'type of fractionation because of deposition of solid asphaltic plugs in the column. In general, each reduced crude feed stock was subjected to the following fractionation conditions toproduce-SAE 40 oils during the operative periods: propane/oil volume ratioj'l/ 1; top tower temperature, about 195 F.; bottom tower temperatureabout F. In the cases of .the two stocks that were not amenable to continuous propane fractionation, the asphalt deposits built up throughout the entire length of the stripping section until virtually complete closure of the column occurred.

The exact mechanism involved in the formation of asphaltic plugs from these low asphalt stocks .is not understood, nor can the usual inspection tests be relied on to indicate which crudes are amenable to this typeof fractionation. The evidence strongly indicates-that-in the case of certain Mid-Continent lube stocks, three phases are formed in the vicinity of the feed plate; namely a propane-rich light oil fraction, an oil-rich propane-oil solution and a semi-solid asphalt-oil solution. These facts have been verified by equilibrium bomb experiments carried out at feed plate conditions, which experiments are the only reliable index as to the amenability of a given reduced crude to propane fractionation. The actual quantity of oil-free asphaltenes involved in inoperative stocks is small, being of the order of 0.05 to about 1.0 per cent by weight as determined in the equilibrium bomb. The seriousness of the problem is further emphasized by the fact that the oil-free asphaltenes found in a completely plugged column amounted to only about per cent of the available free asphalt that could be formed at the feed plate during the operating period. This apparent paradox is resolved on consideration of the fact that the solid plug contains from 50 to 80 per cent oil, thus greatly increasing the bulk of the asphaltic material. It has further been found that in the case of asphalt depositing stocks, variations in temperature, pressure and propane/oil ratio do not materially influence the phase relationships previously set forth.

We have now discovered a procedure whereby all crudes of the Mid-Continent type can be rendered adaptable to propane fractionation. Our process involves dilution of the reduced crude with from 0.5 to 2.0 volumes of propane at temperatures of from about 75 F. to 180 F. followed by simple filtration and subsequent injection of the filtrate into the propane column at the usual feed entry. This type of feed conditioning is particularly advantageous since ordinarily not more than 1 per cent of the feed stock is removed, thus the balance between light and heavy oil required for efficient and economic operation of the propane fractionation process is not materially disturbed. This type of separation is entirely unexpected since literature data on Mid-Continent residua show that no asphalt precipitation is expected at propane/oil ratios below 2/1 and that in cases where the near minimum of propane has been used, the quantity of rejected asphalt phase usually amounts to about 7 per cent by volume based on the reduced crude.

While the pretreatment of feed stock to the propane fractionation unit bears a superficial resemblance to prior art deasphaltizing processes, the end results are significantly different in their relationship to the dependent propane fractionation step. Conventional deasphalting processes require relatively large volumes of propane and yield two liquid phases each of which contain appreciable quantities of propane. This propane unit must be removed from each phase at considerable cost in the form of heaters, stripping columns, condensers, compressors and the like. Furthermore, careful control of the conventional process must be exercised to insure a feed stock of constant composition to the propane column if good fractionation is to be achieved. On the other hand, the process of the present invention provides a feed stock containing usually not more than one volume of propane which does not seriously impair the emciency of the propane fractionation process and and therefore does not require recovery of the propane before entry into the fractionator. This small amount of propane, by virtue of the reduction in viscosity, promotes good dispersion of v the oil and propane phases in the fractionation step. Furthermore, since only a solid asphaltic constituent is removed from the feed stock, the phase relationships entering into the fractionation step are essentially unchanged.

The figure illustrates, diagrammatically, one embodiment of apparatus in which to practice the process of our invention.

Referring now to the drawing, the apparatus consists of a propane storage tank I, a reduced crude oil storage tank 2, a treating tank 3, some separating apparatus 4 and 5, a fractionating apparatus 6, some propane-oil separating apparatus I and 8, and a second propane storage tank 9. Pipes, valves, pumps, coolers and heaters, as shown, will be mentioined in the discussion of the operation of our process which follows.

A low asphalt-content reduced crude lubricating oil stock, hereinafter referred to as the crude oil stock, from tank 2 is passed through a pipe H, and liquid propane from its storage tank I is passed through a pipe l2, and these two materials are mixed upon passage into a mixing device l3. This mixing device 13 may well be a centrifugal mixing pump or other mixing device suitable for mixing of such materials. The mixed oil and propane from mixer l3 passes through a line I4 into the mixing tank 3. In this tank it is intended that the oil-propane admixture be retained for some time so that precipitation of asphalt may be carried to equilibrium. In order to promote constant and thorough mixing of the contents of this tank, a line l5 leads from the bottom thereof to a second mixing device It. Accordingly, tank contents flow through pipe l5, are agitated in mixer I6 and are then passed through a line I! to the top of the tank 3. The rate of this recycle through line I5, mixer 46 and line I! may be at any rate desired and is such that by the time the material leaves tank 3 through a pipe l8 for further treatment asphaltenes or other precipitatable materials are rendered insoluble and adapted to be separated from the remaining liquid solution. This material flows through pipe [8 into separator 4 which may be a filter, a centrifuge or a gravity settler or any other type of separation apparatus adapted for this type of separation. It will be noted that a second separator 5 similar to separator 4 is provided so that in case separator 4 needs to be cleaned or emptied or taken from service for any other reason separator 5 may then be put on stream. Some outlets 2| and 22 are provided for separators 4 and 5, respectively, for withdrawal of solid or semi-solid material following separation from the oil-propane solution. Filtrate from either separator 4 or 5 is passed through a line 23, a heater 24 and through a line 25 into the fractionator 6 at an intermediate point.

In this fractionator B is maintained a propane-rich liquid phase 26 under which in the bottom of the vessel'is maintained an oil-rich phase 21. The oil-rich phase, of course, rests in the bottom of the vessel since it is specifically heavier than the propane-rich phase. Liquid propane is introduced into the fractionator 5 from a line 28 at a point near the base. The propane is introduced into the oil-rich phase through a distributor, not shown, in such a manner that as the propane rises through the oil-rich phase 2'! it will strip propane soluble oils efiiciently therefrom. The globules or upward flowing droplets of liquid propane with their charge of dissolved oil reach the interface 29 and upon entering the propane-rich phase 26 these propane-rich droplets become coextensive therewith. "This propane-rich solution thenslowly rises or flows upward through the iractionator B from the interface 29 between the lower oil-rich phase and the upper propane-rich phase. Charge oil containing some propane in solution which enters fractionator through line flows or settles downward in countercurrent contact with the rising propane solution. From this charge oil the rising propane further strips propane soluble oils. The temperature at the charge oil inlet point may, for example, be maintained at about 190 F., that is, between the limits of 180 F. and 200 F. Preferably, the temperature at the top of the column is higher than that at the feed point, and the feed-point temperature is higher than the temperature at the bottom or the column. The temperature at the top of the column may be-maintained between the limits of 190 F. and 210 F. and the temperature at the bottom of the body of liquid propane containing dissolved oil is between the limits of 170 F. and 190" F. A heat exchanger or heating coil 30 is provided in the top of the fractionator 6 for reflux producing purposes. Since the solubility of certain lubricating oil stock components decreases more rapidly than the solubility of other components, when a higher temperature is maintained in the top of this vessel precipitation of some oil constituents occurs. Thus by maintaining the top of the 'fractionator 6 at the highest temperature within this vessel, the more-aromatic and some higher molecular weight less aromatic, oily constituents tend to precipitate from the propane solution as an--oily liquid phase. This precipitated material in the form of droplets settles downward through the rising solution of propane. Due to the refluxing type of "operation the upper section of this fractionator may be termed a rectifying section, at least as far down as the feed inlet point. From this point at least to the interface 29 the vessel may be termed a stripping section due to the stripping action of the propane solution on the oily feed stock. Oilrich phase containing some propane in solution is withdrawn from the bottom of the fractionator-B through a pipe 3! for transfer to the separator 8. In this separator the dissolved propane may be recovered or separated in any desired manner, such as fractional distillation, from the oily material. The remaining oily material is passed through a pipe 32 to further treatment or storage or any other disposal as desired." The propane from separator 8 is passed through line 33, cooled and condensed in a condenser 34 and as a liquid is passed through a pipe 35 into the propane storage tank 9.

The propane-rich phase containing dissolved oil is removed from the top of the fractionator 6 through a pipe 4| and is conducted into the separator I. In this separator the propane is separated from the oil and the propane passed through a pipe 42, condensed in condenser 43 and is finally passed on through a pipe 44 into the propane storage tank 9. Overhead oil product separated from the propane in separator 1 is removed therefrom through a line 45 and may be passed to further treatment or storage or other disposal as desired.

The separators I and 8 may be flash separators supplemented by stills for separating final traces of propane from the oils or the separators may merely be stills withous flash separators. They may, however, be any type of sepin solution. which leaves Cir 6 arator which is suitable for the separation of propane from the oils.

The oil-rich phase containing some propane the fractionator 6 through line 3'! may in place of passing to the propane separator 8 be passed to a second oil fractionation step similar to that carried out in fractionator 6. In such a second fractionation step, a higher viscosity overhead oil may be fractionated. Any number of subsequent fractionation steps may be carried out on the material from'pipe .3! if desired.

In a similar manner the propane solution containing oil fiowing through pipe 41 may be conducted to. subsequent fractionation steps if desired prior to separation of the propane from the oil.

A portion of the propane accumulated in tank t may then be passed through a pipe 51' into the original run storage tank I. The remainder of the propane from line 9 is accordingly passed through arecycle line 52, through a heater 53 and on through line 28 into the bottom portion ofthe fractionator 6.

The ratio of propane to the low asphalt content reduced crude lubricating oil charged to the mixer l3 may vary between the limits of 0.521 to 2:1. The temperature at the point of mixing of these two stocks may vary from 75 to 180 F., and likewise, this same temperature is maintained during the entire time of contact of the propane and oil stock in vessel 3. A preferred volume ratio of propane to oil stock in vessel 3 is 111. It will be realized, however, that this'ra-tio may be varied on either side of the 1 :1 ratio depending upon the particular oil stock being treated. The ratio should be so selected that the precipitating action in vessel 3 will be such that the asphaltic material will be converted into an easily filterable form.

In one particular operation the temperature in the mixer 13 and the mixing tank 3 is F. with a 15:1 propane-to-oil ratio when treating a Mid-Continent crude oil stock containing 2% of asphalt as determined by the aforementioned A. S. T. M. method. The temperature in the bottom of the fractionator 6 is maintained at F. A temperature of F. at the feed point and a temperature of 200 F. in the vicinity of the heater 3c are also maintained. When propane fractionating the above mentioned reduced crude oil stock under these temperature conditions and with a total propaneto-oil volume ratio of '7 based on the volume of oil charged to mixer l3, an SAE 40 viscosity oil stock is recovered through line 45. Operation may be continued for an extended period of time without deposition of an asphalt plug in the fractionating column.

In case the separators 4 and 5 are filters, the filter beds may comprise a bed of sand, excelsior, metal turnings or the like.

It will be obvious to those skilled in the art that many variations and alterations in the operating conditions may be made and yet remain within the intended spirit and scope of our invention. The apparatus and materials of construction thereof may for the most part be selected from among materials and apparatus commercially avail-able. lowever, when apparatus is used in such a system, for example, a liquid-propane system, the vessels and pipes and the like will need to be constructed of heavy material to withstand such relatively large pressures as will be required to maintain the propane in the liquid state, or in an easily condensable state. Pressure in the fractionating column may be about 650 p. s. i. The propane in this system will be in a liquid state in all points of the apparatus excepting in the separator 8 and pipe 33 and in separator T and pipe 42 in case separators I and 8 are stills.

We claim:

1. In the propane fractionation of lubricating oil constituents from low asphalt-content reduced crude lubricating oil stocks with liquid propane wherein solid asphaltic material precipitates upon initial contact with the propane and adheres to and plugs the fractionating apparatus in the region adjacent and below the raw feed entry point, a process for continuously carrying out this fractionation operation without precipitating said asphalt and plugging said apparatus, comprising admixing with said oil stock from to less than 2 volumes liquid propane per volume of said oil stock passing the admixture into a mixing zone, circulating said admixture in said mixing zone to mix thoroughly said propane and said reduced crude oil stock to precipitate solid asphaltic material as a solid phase and to maintain said solid phase suspended in a liquid asphalt oil phase containing said propane in solution, passing the circulated admixture containing said solid asphalt phase in suspension to a separation zone, separating therein said solid phase from said liquid asphalt-oil phase containing propane in solution, removing the separated solid phase as a byproduct of the process, passing said liquid asphalt-oil phase containing propane in solution into a body of liquid propane containing dissolved oil in a treating zone, maintaining a body of liquid asphalt and oil containing dissolved propane below said body of liquid propane containing dissolved oil in said treating zone, said bodies meeting at a common liquid-liquid interface, introducing a stream of liquid propane into said body of liquid asphalt and oil containing dissolved propane, removing liquid asphalt and oil containing dissolved propane from the bottom of said zone, removing propane containing dissolved oil from the top of said treating zone and recovering the oil from the last mentioned propane as a second product of the process.

2. In the operation according to claim 1, wherein said separation zone comprises a pair or separation zones, and alternately passing said circulated admixture containing said solid asphaltic material in suspension in said liquid asphalt-oil phase containing propane in solution into one zone of said pair of zones and therein separating said solid asphaltic material from said liquid asphalt-oil phase, and simultaneously removing previously separated solid asphaltic material from th other zone of said pair of zones as said byproduct of the process.

3. The method of claim 2 wherein the temperature of the top of the treating zone is maintained between the limits of 190 F. and 210 F., the temperature at the feed point to said treating zone is maintained between the limits of 180 F. and 200 F., and the temperature at the bottom of said body of liquid propane containing dissolved oil is maintained between the limits of F. and 190 F.

4. In the operation according to claim 1 wherein the admixing operation of to less than 2 volumes of liquid propane per volume of oil stock is carried out between the temperature of 75 F. and F.

5. The method of claim 1 wherein the volume of liquid propane introduced into said body of liquid asphalt and oil containing dissolved propane in said treating zone is between the limits or 3 and 10 times the volume of low asphalt-content reduced crude lubricating oil stock to the process.

WILLIAM N. AXE. ALBERT N. DE VAULT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

1. IN THE PROPANE FRACTIONATION OF LUBRICATING OIL CONSTITUENTS FROM LOW ASPHALT-CONTENT REDUCED CRUDE LUBRICATING OIL STOCK WITH LIQUID PROPANE WHEREIN SOLID ASPHALITIC MATERIAL PRECIPITATES UPON INITIAL CONTACT WITH THE PROPANE AND ADHERES TO AND PLUGS THE FRACTIONATING APPARATUS IN THE REGION ADJACENT AND BELOW THE RAW FEED ENTRY POINT, A PROCESS FOR CONTINUOUSLY CARRYING OUT THIS FRACTIONATION OPERATION WITHOUT PRECIPITATING SAID ASPHALT AND PLUGGING SAID APPARATUS, COMPRISING ADMIXING WITH SAID OIL STOCK FROM 1/2 TO LESS THAN 2 VOLUMES LIQUID PROPANE PER VOLUME OF SAID OIL STOCK PASSING THE ADMIXTURE IN A MIXING ZONE, CIRCULATING SAID ADMIXTURE IN SAID MIXING ZONE TO MIX THROUGHLY SAID PROPANE AND SAID REDUCED CRUDE OIL STOCK TO PRECIPITATE SOLID ASPHALTIC MATERIAL AS A SOLID PHASE AND TO MAINTAIN SAID SOLID PHASE SUSPENDED IN A LIQUID ASPHALTOIL PHASE CONTAINING SAID PROPANE IN SOLUTION, PASSING THE CIRCULATED ADMIXTURE CONTAINING SAID SOLID ASPHALT PHASE IN SUSPENSION TO A SEPARATION ZONE, SEPARTING THEREIN SAID SOLID PHASE FROM SAID LIQUID ASPHALT-OIL PHASE CONTAINING PROPANE 